The present invention relates to a modifier for devulcanization of cured elastomers and to a method for devulcanization of cured elastomers utilizing the same.
It is known that a spatial network of sulfur-cured elastomers has three types of chemical bonds:
Cxe2x80x94C, Sxe2x80x94C, Sxe2x80x94S, i.e. carbon-carbon bonds, sulfur-carbon bonds and sulfur-sulfur bonds.
The devulcanization is conducted by means of destruction of the inter-chain cross bonds Sxe2x80x94C and Sxe2x80x94S in the elastomer, which bonds are weaker than main-chain Cxe2x80x94C bonds.
Several methods for devulcanization of cured elastomers, mainly of rubber, are known at present.
In some of these methods, stressed cured rubber is treated by high temperature (U.S. Pat. Nos. 5,883,140, 5,731,358) or by microwave action, accompanied by heating the material (U.S. Pat. No. 4,104,205).
Deep destruction of the main chains accompanies all the high-temperature methods and thus leads to the partial loss of physical and mechanical properties of secondary vulcanizates.
Other methods (U.S. Pat. No. 5,258,413) involve ultrasonic treatment of rubber through a liquid medium.
This method requires further separation of finished material from liquid and for this reason is too complicated.
Devulcanization by means of biotechnological reactions is described in several patents (U.S. Pat. Nos. 5,677,354, 5,798,394, 5,891,926).
This method require complicated equipment, large floor-space and excessive operating time.
All the above-mentioned methods, do not use ambient solid-phase modifiers. Modifiers are solid substances, generating a specific agent, which initiates destruction of inter chain cross bonds Cxe2x80x94S and Sxe2x80x94S bonds and thus brings about the devulcanization of cured elastomers.
It is well known that the devulcanization process is aimed at obtaining secondary vulcanizates with a minimal loss of main physical and mechanical parameters (tensile strength, elasticity, elongation etc.). One of the main factors, which are negatively influent at these parameters, is a destruction of main-chain bonds.
An ambient solid-phase modifier for devulcanization is known (see, e.g., EP 0 690 091 A1). This modifier includes two substances, which generate protons during reaction and initiate a proton exchange with the polymer structure of a pre-stressed cured elastomer. In response to this interaction the elastomer is devulcanized. The process is conducted at smooth 2-roll mill under conditions of stressing the devulcanized material.
There are however disadvantages inherent in this method. Having a short term of active life, the proton penetrates into the material only to a small depth. This results in a need for fine grinding of the cured rubber (up to 0.4 mm and less) and correspondingly reduces cost-efficiency of this method.
Furthermore, the proton does not display good selective action to different bonds and along with the attack of Cxe2x80x94C and Sxe2x80x94S cross bonds, it attacks also Cxe2x80x94C bonds and initiates their partial destruction. This brings about reduction of the physical and mechanical properties of secondary vulcanizate. According to the data of said patent, secondary devulcanizate from tire scraps show tensile strength at a level 11-13 MPa and elongation at break at a level 300-350%, which means retention of only 62-70% of its values as compared with primary (fresh rubber) vulcanizate.
An object of the present invention is to create an ambient solid-phase modifier and to develop conditions for its use, so that in the course of devulcanization components of the modifier will generate a specific agent, which selectively destroys inter-chain cross bonds and maximizes the retention of main-chain bonds while exhibiting a maximum term of active life. This modifier and mode of its use are to provide best permissible physical and mechanical properties of secondary vulcanizate and to obviate the need of fine grinding a vulcanized rubber before its devulcanization.